Apparatus that are designed to measure one or more characteristics of a population of particles, such as biological cells, have become quite common in laboratory environments. Such apparatus are available for analyzing a wide range of particle types using a variety of different analysis methods. For example, data relating to a particular cellular characteristic may be acquired through direct observation of treated biological cell populations. This method is frequently used to diagnose hematological anomalies and diseases. Another type of analysis system that acquires biological and/or chemical data in an indirect manner employs a population of man-made particles. Generally stated, the prefabricated particles are in the form of spheres. The spheres include non-reactive cores having reactive surface coatings. The surface coatings are chosen to react with a target chemical constituent of an analyte in a known manner to thereby alter the characteristic, such as fluorescence, of the spheres. The spheres are introduced to an analyte and the resulting reaction (or failure to react) is detected by the analysis system.
U.S. Pat. No. 5,125,737, to Rodriguez et al. describes a flow cytometer in which a stream of particles is passed into and through a point focused beam of electromagnetic radiated energy. The '737 patent is incorporated herein by reference. One embodiment of the apparatus is shown in FIG. 1. The apparatus comprises an elongated, cylindrical member, forming a flow cell 10. The flow cell 10 can be formed from any optically transparent material, for example, fused silica, quartz, or sapphire. The interior portion of the flow cell 10 is cylindrical throughout its length, except for a narrowed or necked-down aperture 12 through which a biological cell sample is passed as a focused stream 14. The exterior walls 15 of the flow cell 10 are cylindrical and include an optical flat 16. A lens system 18 focuses a beam 20 of electromagnetic energy, preferably from a laser 22, into a spot at the aperture 12. A photodetector assembly 24, acting as a scattered radiation receptor, is positioned in a plane orthogonal to and centered on the commission axis 26 of the laser radiated electromagnetic energy.
The photodetector assembly 24 is comprised of an electromagnetic sensor 25 with a mask 28 and a beam dump 30 (an obstruction for removing unwanted laser light). The mask 28 can be of a circular, elliptical, or other shape, as required to obtain equivalent light scattering information from flow cells 10 of different architectures. The mask is oriented coaxial with the laser light beam 20. The beam dump 30 extends horizontally across the photodetector assembly 24 facing the laser beam as shown. The beam dump 30 can be slightly angularly fanned out from the center to provide a cleaner signal-to-noise output.
In operation, electromagnetic radiation emitted by laser 22 passes through the lens system 18 and, therefrom, through the particle stream passing through flow cell 10. The electromagnetic radiation is scattered by the particles of the stream 14 into a plurality of beams 32. The magnitudes of the beams 32 at various scattering angles are detected by the photodetector assembly 24. Photodetector assembly 24, in turn, provides electronic signals corresponding to the magnitudes of the beams 32 to subsequent electronic circuits for further processing, conversion and data analysis.
The position of the photodetector assembly 24 with respect to the other elements of the apparatus has now been found to be an important aspect of the overall accuracy of the foregoing system. Until now, the accurate positioning of the photodetector assembly 24 has been achieved using fixed sensor support frames that are manufactured to extremely tight tolerances. Such tight manufacturing tolerances are both costly and sometimes difficult to achieve. Accordingly, an alternative to this fixed sensor support approach is desirable.